1. Field of the Invention:
This invention relates generally to fail-safe systems for respirating gas delivery devices which provide pulsed doses of respirating gas to a patient.
More specifically, this invention relates to fail-safe systems which ensure a continuing flow of respirating gas to a patient in the event that a pulse-dose respirating gas delivery system fails to operate properly.
2. Description of Related Art:
It has long been conventional practice to administer supplemental oxygen to patients suffering from chronic obstructive pulmonary diseases and other respiratory ailments. Devices commonly used for oxygen administration deliver a constant flow of oxygen at a fixed rate to a mask placed over the patient's nose and mouth or through a cannula which terminates in nares inserted into the patient's nostrils.
Constant flow devices waste a substantial portion of the oxygen because that oxygen provided during the exhalation and pause phases of the patient's respiratory cycle cannot be used. Consequently, devices have been developed to conserve oxygen by regulating the oxygen flow, turning it on and off, in synchronization with the respiratory cycle. Typically these devices operate by sensing the beginning of an inspiration and delivering pulses or doses of oxygen at a relatively high rate beginning at the start of inspiration but lasting for only a small part of the inspiration period.
The sensors and control circuitry for such devices are ordinarily powered by electricity and require a current source such as a battery. Also, the valves used to control oxygen flow are usually electrically operated solenoid valves. Delivery of oxygen doses will cease in the case of malfunction of the sensor or the control circuitry or the failure of the power source. A delivery failure can have serious adverse effects upon a patient and may even become life-threatening. Consequently, pulse dose oxygen delivery systems typically have some means for switching to a continuous delivery mode upon need.
Pulse dose oxygen systems known in the prior art fall generally into two types; one type employing rate-time metering and the other type employing volumetric metering. An example of a volumetric metering system is shown by appliant's U.S. Pat. No. 4,705,034. The demand oxygen controller developed by Dr. Gerald Durkan, represented for example by his U.S. Pat. Nos. 4,457,303 and 4,462,398, is of the rate-time metering type. It employs in its commercial embodiment a manually operated selector switch which changes the gas delivery between a pulsed and a continuous flow mode. The continuous flow mode requires no electrical power.
Another example of a pulse dose oxygen delivery system employing rate-time metering is the Puritan-Bennett device described in U.S. Pat. No. 4,706,664. That apparatus is designed to revert to conventional, continuous flow operation upon a power failure or circuit malfunction. Reversion to continuous flow operation is accomplished by mechanically biasing an electrically operated solenoid valve so that it will move to an open position whenever the solenoid is de-energized. That arrangement requires the solenoid to have sufficient power to overcome the mechanical biasing force as well as to change the valve position. The solenoid must also remain energized so long as the valve is in the closed, or flow blocking, position.
The back-up systems of the prior art, allowing for a change from pulse dose delivery of oxygen to a continuous delivery mode in case of malfunction or power failure, all have practical disadvantages. The Durkan system requires that the patient recognize the malfunction and physically change the position of a selector switch. The Puritan-Bennett system, while functioning automatically, greatly increases the power consumption of the unit because of the need to maintain a mechanically biased solenoid valve in an energized state.